Frequency converters are used in order to convert an electromagnetic signal at one frequency to another frequency. The frequency conversion can be either up-conversion or down-conversion, i.e. a signal at a certain frequency can be shifted either to a higher or a lower frequency. Frequency converters are used in such systems as, for example, radio communications systems and radar systems.
A traditional frequency converter uses a mixer which has as its input signals the signal whose frequency it is desired to shift and a signal at another frequency, and provides as output the mixing product of the input signals. This product comprises so called sum and difference signals, i.e. signals at frequencies which are the sum and the difference of the frequencies of the input signals.
The parameter usually referred to as conversion gain, defined as the ratio of the power (or, alternatively, voltage) of the output signal to the power (or alternatively, voltage) of the input signal whose frequency is shifted in the frequency converter, is a parameter which it is naturally a desire to improve on in frequency converters.